It is well-known epoxy resins may be used to treat surfaces (e.g. concrete, metal, electrical components, and sheetrock) to protect against corrosion and other forms of wear and tear caused by everyday use and the environment. The epoxy resins generally contain a plurality of epoxy or oxirane groups which react with a curing agent to form a network or significantly cross-linked system.
Curing agents are to be distinguished from compounds referred to herein as chain extension agents. As used herein, a “chain extension agent” is meant to refer to a compound which has two (2) sites capable of reaction with epoxy groups. During polymerization, a chain extension agent will generally become lodged between epoxy resin chains, extending the same with little cross-linking occurring. In comparison, a “curing agent” refers to a compound capable of catalyzing polymerization of an epoxy resin with substantial networking or cross-linking.
In many applications, it's desired that the cured epoxy product exhibit a relatively high glass transition temperature (Tg). One commonly used method for obtaining a higher glass transition temperature is through the use of multifunctional epoxy resins such as those described in U.S. Pat. Nos. 4,559,395, 4,645,803, 4,550,051, 4,529,790, 4,594,291, 2,947,726, 2,971,942 and 2,809,942. However, multifunctional epoxy resins are typically not very tough or ductile and are therefore undesirable in some applications. In addition, if large concentrations of polar groups are incorporated into the resin in order to achieve a high thermal resistance, the cured product may exhibit poor moisture resistance properties.
One method for improving toughness and flexibility is through the use of chain extension agents, such as bisphenol A, which can be incorporated into the epoxy resin prior to curing. While the resulting cured product generally exhibits a relatively high degree of curing and toughness or ductility, the cured product will also tend to exhibit a relatively low glass transition temperature because of a low cross link density. The epoxy resin described in U.S. Pat. No. 4,980,234 which is based on the diglycidyl ether of 9,9-bis(4-hydroxyphenyl)fluorene, is taught to provide a material having a high glass transition temperature and flexural modulus combined with a low moisture resistance when cured.
It is an object of the present invention to provide new epoxy resin compositions from which cured epoxy products having even higher glass transition temperatures combined with excellent mechanical properties and low moisture absorption can be obtained.